The letter from Jon Adler, the department's director of the Bureau of Justice Assistance, says the department is concerned that Oakland's policy may violate a federal law that bars local governments from preventing their employees from communicating with federal immigration agents.

"This is another tactic by this administration to continue to threaten cities. to bully us and strike fear into the heart of vulnerable communities," Mayor Schaaf told KTVU on Friday. "When we adopted our sanctuary policies, we researched the federal law. We adopt policies that we believe are absolutely in compliance with federal law."

Compliance with the law, known as Section 1373, is a condition of Justice Department grants to local governments under a program known as the Edward Byrne Memorial Justice Assistance Program.

Similar letters previously have been sent to other cities and counties, including San Francisco, Berkeley, Fremont and Watsonville and Contra Costa, Monterey, Santa Clara and Sonoma counties.

"Every time this administration has threatened either deny or claw back grant money; even criminal justice grant money, the courts have said no and that's cities acting within their absolute legal right," Schaaf said.

U.S. Attorney General Jeff Sessions said in a statement, "When cities and states enact policies that thwart the federal government's ability to enforce federal immigration law, they choose to place the protection of criminal aliens over the safety of their communities."

"The Justice Department will not tolerate this intentional effort to undermine public safety and the rule of law, and I continue to remind all jurisdictions to reconsider policies that put their residents in harm's way," Sessions said.

Schaaf countered, "We will continue to push back against this ugly myth that immigrants and even undocumented workers or residents in our cities do not make our city safer," when she spoke to us on Friday.

Adler's letter asks for Oakland to submit a response by May 14 explaining whether Oakland has laws, policies or practices that violate Section 1373.

"We will respond to this letter," said Schaaf. "We will also continue to believe that our democratic system--three branches of government. particularly that judicial branch, will continue to put a check on this administration."

Adler wrote, "The department is concerned that the city of Oakland's laws, policies or practices may violate Section 1373, or, at a minimum, may be interpreted or applied in a manner inconsistent with Section 1373."

However, Adler said, "Please be advised that the department has not made a final determination regarding the city of Oakland's compliance with Section 1373" and the letter "does not constitute final agency action."

A spokesman for Schaaf said she has forwarded the letter to City Attorney Barbara Parker to review it. A spokesman for Parker wasn't immediately available for comment.

Oakland City Attorney Barbara Parker responded Friday in a statement that said, "We are reviewing the letter from the Department of Justice. Oakland is proud to be a sanctuary city and we are in compliance with federal immigration law. Our sanctuary city policy encourages the community to work with police and helps law enforcement to investigate and prevent crimes in our city. We will continue to focus our resources on fighting crime, rather than tearing apart Oakland families and making our city less safe. We will respond to the letter at the appropriate time."